The present invention relates to an ultrasonic transmitter for generating incoherent, or diffuse, ultrasonic radiation.
Ultrasonic acoustic imaging techniques, in contradististinction to conventional echo processes for medical diagnosis, serve to provide optical representations of differences in attenuation of acoustic energy in the human body.
For this purpose, the subject is penetrated, or insonified, by an ultrasonic acoustic wave and a suitable lens with large aperture images, or focusses, the ultrasonic information on a detector array. Such a method is disclosed, for example, by J. F. Havlice et al. in Acoustical Holography, Volume 7, edited by L. W. Kessler, Plenum Press, 1977, at pages 291-305.
Since it was found that a coherent image made with but one ultrasonic transmitter was unable to furnish reliable images for diagnostic purposes, Havlice et al. employed, in a further development of the transmission method, twenty to thirty independent ultrasonic transmitters and thus realized a partially spatially incoherent insonification of the subject.
The resulting ultrasonic images are of usable quality, particularly for the imaging of tendons and vessels in extremities. However, for images in the upper abdominal region through the body, the long path traversed has an adverse influence on the quality of the image.
In principle, an ultrasonic transmission arrangement includes a transmitting member with condenser lens in front of the subject and a receiving member with objective lens behind the subject.
The transmitting member for diffuse insonification includes a plurality of sound sources whose emitted sonic fields are statistically independent of one another. Due to the coherence conditions known in optics, regions with an area F.sub.El must be considered to be spatially coherent elementary sources according to equation (1). EQU F.sub.El =.lambda..sup.2 A.sup.2 /F.sub.Ap ( 1)
where
.lambda.=wavelength of the ultrasonic radiation
A=transmitter--condenser distance, and PA1 F.sub.Ap =area of the condenser lens aperture.
It would therefore make no sense to further reduce the area of the elementary sources. The maximum number, N.sub.max, of mutually incoherent elementary sources in an expanded source then results from equation (2). EQU N.sub.max =F.sub.Source /F.sub.El =F.sub.Ap F.sub.Source /.lambda..sup.2 A.sup.2 ( 2),
where F.sub.Source is the area of the expanded source.
In order to realize as incoherent as possible an insonification with an expanded source, N elementary sources of the size indicated in equation (2) should be used, where N is a large number. Each one of these individual sources produces an image in the detector plane, the image information of interest always being the same and the noise resulting from scattering or from interference effects changing from source to source. From statistical considerations it follows that the signal-to-noise ratio which is proportional to the square root of the number N of elementary sources increases up to a maximum value for which N has the value given by equation (2). For a conventional transmission system, the following parameters apply: f=2 MHz (.lambda.=0.75 m), A=50 cm, source diameter=condenser lens aperture diameter=20-25 cm.
From this, it follows that N.sub.max .congruent.10.sup.4 with respect to the transmitter area.
A system having the above-mentioned parameters should thus include, for diffuse insonification, approximately 10.sup.4 independent individual transmitters so as to obtain an image which is as free from interference as possible. The system produced by Havlice et al. uses, as a maximum, 30 independent individual transmitters with each ultrasonic transmitter having its own actuating unit and amplifier unit. An expansion of the number of transmitters by 1 or 2 orders of magnitude based on this system appears impossible.